This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or tundish/distributor, from which it flows through a metal delivery nozzle located above the nip, which directs the molten metal to form a casting pool supported on the casting surfaces of the rolls above the nip. This casting pool is typically confined at the ends of the casting rolls by side plates or dams held in sliding engagement adjacent the ends of the rolls.
In casting thin strip by twin roll casting, the metal delivery nozzles receive molten metal from the movable tundish and deposit the molten metal in the casting pool in a desired flow pattern. Previously, various designs have been proposed for delivery nozzles involving a lower portion submerged in the casting pool during a casting campaign, and having side openings through which the molten metal is capable of flowing laterally into the casting pool outwardly toward the casting surfaces of the rolls. Examples of such metal delivery nozzles are disclosed in U.S. Pat. No. 5,857,514 and U.S. Pat. No. 6,012,508. In prior art metal delivery nozzles, there has been a tendency to produce thin cast strip that contains surface defects and associated microcracking from uneven solidification at the chilled casting surfaces of the rolls.
The present invention provides a metal delivery nozzle that is capable of substantially reducing and inhibiting such surface defects and microcracks. By testing, we have found that a major cause of such defects is premature solidification of molten metal in the regions where the casting pool meets the casting surfaces of the rolls, generally known as the “meniscus” or “meniscus regions” of the casting pool. In these regions, if solidification occurs before the molten metal has made contact with the roll surface, irregular initial heat transfer can occur between the metal shell and the casting roll, resulting in formation of surface defects, such as depressions, ripple marks, cold shuts and/or microcracks. The temperature of the metal in the surface region of the casting pool between the rolls tends to be lower than that in the incoming molten metal. If the temperature of the molten metal at the pool surface in the region of the meniscus becomes too low then surface cracks and “meniscus marks” (i.e., marks on the strip caused by the meniscus freezing while the pool level is uneven) are likely to occur.
One way of dealing with such surface cracks and meniscus marks has been to increase the temperature of the incoming molten metal from the delivery nozzle, so that molten metal reaches the casting surfaces of the rolls before reaching solidification temperatures. Another approach has been to cause the incoming molten metal to be delivered relatively rapidly into the meniscus regions of the casting pool directly from the delivery nozzle. This avoided the tendency for premature solidification of the metal before it contacts the casting roll surfaces. This approach has been more effective in avoiding surface defects in the cast strip. Examples of this approach are to be seen in U.S. Pat. No. 5,875,514. This approach allows for cast strip without the formation of surface defects and cracks.
Nevertheless, the formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams have been observed. The rate of heat loss from the melt pool is higher near the side dams due to conductive heat transfer through the side dams to the casting roll ends. This localized heat loss near the side dams has a tendency to form “skulls” of solid metal in that region, which can grow to a considerable size and fall between the casting rolls and causing defects in the cast strip. An increased flow of molten metal to these “triple point” regions, the regions near the side dams, have been provided by separate direct flows of molten metal to these triple point regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511. Increased heat input to these regions has inhibited formation of skulls.
U.S. Pat. No. 5,857,514 discloses a method and apparatus in which molten metal is delivered to the delivery nozzle in a trough closed at the bottom. Side openings are provided through which the molten metal flows laterally from the nozzle into a casting pool in the vicinity of the casting pool surface. The flow of molten metal into the casting pool was improved; however, unevenness in metal flow adjacent the casting roll surfaces tended to cause surface defects and surface cracks in the cast strip.
The present invention provides an improved delivery nozzle and method of casting thin strip with the delivery nozzle. Disclosed is a method of casting metal strip comprising:                (a) assembling a pair of casting rolls laterally disposed to form a nip between them;        (b) assembling an elongated metal delivery nozzle extending along and above the nip between the casting rolls, with at least one segment having opposing side walls and end walls, and an inner trough extending longitudinally between the side walls and forming passages between the side walls and the inner trough and communicating with side outlets adjacent bottom portions,        (c) introducing molten metal through the elongate metal delivery nozzle to form a casting pool of molten metal supported on the casting rolls above the nip, such that molten metal is caused to flow into the inner trough of the delivery nozzle, from the inner trough through the passages between the inner trough and sidewalls, and from the passages through the side outlets in a substantially lateral direction into the casting pool, and        (d) counter rotating the casting rolls to deliver cast strip downwardly from the nip.        
Each segment of the metal delivery nozzle may be assembled with at least one partition extending between the side walls, and the passages between the inner trough and side walls extending between the partitions or between a partition and end wall.
Each segment of the metal delivery nozzle may be assembled with inner trough and side walls joined with a shoulder portion therebetween and the passages between the side walls and the inner trough formed by a plurality of holes through the shoulder portion.
Each segment of the metal delivery nozzle may be assembled with the inner trough and side walls in separate pieces, pinned together with ceramic pins. Protrusions may extend into the passages from the inner trough or side wall, or both, to cause turbulence in the molten metal flowing through the passages. The protrusions may be in at least two offset rows extending from the inner trough or side wall on one or both sides of the passages.
Also disclosed is a metal delivery apparatus for casting metal strip comprising a metal delivery nozzle having at least one elongated segment, each segment having opposing side walls and end walls, an inner trough extending along the side walls to form passages between the side walls and the inner trough, and communicating with side outlets adjacent bottom portions of the segments of the delivery nozzle extending along the segment, such that molten metal is capable of flowing into the inner trough, from the inner trough through the passages between the inner trough and sidewalls, and exit the delivery nozzle through the side outlets in a substantially lateral direction into a casting pool. Each segment of the metal delivery nozzle may be assembled with at least one partition extending between the side walls, and the passages between the inner trough and side walls and the related side outlets extending between the partitions or between a partition and an end wall.
Alternatively, disclosed is a metal delivery apparatus for casting metal strip comprising a metal delivery nozzle having at least one elongated segment, each segment having opposing side walls and an inner trough extending along the side walls to form a shoulder portion between the side walls and the inner trough, and a plurality of holes extending through each shoulder portion and communicating with side outlets adjacent bottom portions of the segments of the delivery nozzle extending along the segment, such that molten metal is capable of flowing into the inner trough, from the inner trough through the holes between the inner trough and sidewalls, and exit the delivery nozzle through the side outlets in a substantially lateral direction into a casting pool.
Alternatively, the metal delivery apparatus for casting metal strip may comprise a metal delivery nozzle having at least one elongated segment, each segment comprising an outer piece forming opposing side walls and end walls, and an inner trough forming an inlet to receive molten metal and passages between the side walls and the inner trough, such that molten metal is capable of flowing from the inner trough through the passages between the inner trough and sidewalls, and from the passages exiting the delivery nozzle through the side outlets in a substantially lateral direction into the casting pool.
Each segment of the metal delivery nozzle of metal delivery apparatus may be assembled with the inner trough and outer portion pinned together with ceramic pins. In addition, protrusions may extend into the passages from the inner trough or side wall, or both, to cause turbulence in the molten metal flowing through the passages. The protrusions may be in at least two offset rows extending from the inner trough or side wall on one or both sides of the passages.
In each embodiment of both the improved delivery nozzle and method of casting steel strip with the delivery nozzle, the inner trough dissipates a substantial part of the kinetic energy present in the molten metal by reason of movement through the metal delivery system from the tundish to the delivery nozzle. In addition, the resistance provided in the movement of the molten metal from the inner trough through the passages to the side outlets further reduces the kinetic energy in the molten metal before reaching the casting pool. As a result, a more uniform and more quiescent flow of molten metal is provided to the casting pool to formation of the cast strip.
Other embodiments of the invention will be apparent as following detailed description of the drawings and the claims proceeds.